LTO : how to specify number of chunks with option -flto-partition=balanced

[Richard Biener]

On Fri, May 31, 2013 at 9:21 AM, Swati <swatirathi@cse.iitb.ac.in> wrote:
> For -flto-partition=balanced, if user does not specify the --param
> lto-partitions and --param lto-min-partition,
> will GCC partition the program?
> If yes what is the criteria?

It will partition the program into lto-partitions partitions unless
the partitions
would not be of at least lto-min-partition size.

> Where can we get more help on using the --param lto-partitions and --param
> lto-min-partition?

The default should be good.  If you have a machine with very very many
CPUs then increasing --param lto-partitions may help compile-time.

> We want to know how the partitions are formed using these options.
> Cases
> 1. when we specify --param lto-partitions=3 -> no partitions are formed.

Using a single partition results in a partition of size less than
lto-min-partition so only one partition is used (to avoid creating very very
small partitions).

> 2. when we specify --param lto-partitions=3 --param lto-min-partition=5 -> 3
> partitions are formed
> 3. when we specify --param lto-min-partition=5 -> 7 partitions are formed

This means your program has size < 8 * 5 and thus at most 7 partitions
of minimum size 5 are formed.

> We understand cases 2 and 3.
> However in case 1 why no partitions are formed?

Because the default for lto-min-partition is 1000.

> Is it that when lto-min-partition is not specified, partitions will not be
> formed?

No, it depends on the size of the input program.  LTO tries to create
--param lto-partitions number of partitions unless the partitions would
become smaller than --param lto-min-partition, in which case the number
of partitions is reduced until the partitions are at least of --param
lto-min-partition
size.

Richard.

> Regards,
> Swati
>
>
> On Friday 24 May 2013 02:06 PM, Richard Biener wrote:
>>
>> On Fri, May 24, 2013 at 9:14 AM, Swati<swatirathi@cse.iitb.ac.in>  wrote:
>>>
>>> Hello,
>>>
>>> -flto-partition=balanced is used to specify partitioning into equally
>>> sized
>>> chunks.
>>> Is there an option to specify size or number of chunks?
>>
>> There are the --param lto-partitions and --param lto-min-partition params.
>>
>> Richard.
>>
>>> Regards,
>>> Swati
>
>